Several videos from today's bombing at the Boston marathon capture the digital clock positioned over the finish line. According to the clock, the first explosion erupts at four hours, nine minutes from the race's start.

It is certainly too early to distinguish useful information from distraction. But it is worth noting that a finishing time from four to four and a half hours is a common benchmark for marathoners. According to RunTri, a popular marathon training website, the average runner finished last year's Boston course in four hours, 18 minutes.

Whether by intention or coincidence, today's attack at the finish line appears to have coincided with the moment when, statistically, the largest number of runners—and presumably their friends and family—would be nearby.